This invention concerns a coupling flange system providing interconnection between hollow or similar transmission shafts connecting together two power devices or machines, respectively the motor and the receiver, distant from one another.
A typical case is power transmission in a rotary wing aircraft like a helicopter, connecting the main rotor transmission box to the anti-torque tail rotor transmission box. Because of the distance between the two boxes (several meters), transmission comprises several aligned shafts supported by bearings along the transmission line and coupled together, and with the respective drive and receiver shaft. To do this, the ends facing the two shafts, extending one another, are equipped with flanges which, by a link connecting them together, form a coupling or coupling sleeve as such, which link between the flanges in this example is elastic and, more particularly to compensate for the slight deformations and aligning defects that may appear.
Naturally the invention is not confined to this particular application to the power transmission of a helicopter and could be used in other technical areas, as long as power or torque has to be transmitted by rotation between two devices, respectively the driving and receiving devices.
Generally, each flange of a coupling is fitted around the outer surface of the tubular end of a hollow shaft either by bonding and riveting or bolting, or by welding.
Although widely used, these two solutions nevertheless have drawbacks that cannot be left out of consideration.
In the first case, the weak spot is at the holes for riveting or bolting arranged radially at the shaft end, leading to local over-stressing that is always detrimental, in particular for the dynamic forces encountered on a helicopter (change of power level related to tail rotor maneuvering).
In the second case, welding causes a drop in the fatigue strength of the materials because of the annealing of the welding parts, making it necessary to allow for extra-thickness in the welding zone.
Furthermore, once attached to the ends of the shafts, the flanges cannot be removed (weld) or are almost irremovable (bonding and riveting-bolting) so that whenever a problem occurs, by exercise in a bearing (previously mounted on the shaft before the attachment of the flanges), a balancing device or even at a flange, the complete assembly concerned, the “shaft-end flanges-bearings” needs to be replaced by disassembly of the corresponding coupling flanges and replaced by a new assembly. Naturally, this results in high maintenance costs and the lasting immobilization of the helicopter.
In addition, to associate more generally a shaft and a hub or two shafts with one another, in a known embodiment, a friction link is used, for instance a set of two nested rings having combined external and internal conical surfaces. For instance, U.S. Pat. No. 5,067,847 describes such an embodiment in which the conical assembly is mounted between a solid shaft and a hub of a part. The inner conical ring works with the solid shaft and the outer ring with the hub so that the relative end movement of the internal ring on the shaft, by a controllable device, pulls on the outer ring which is opened radially by the joint operation of the combined conical surfaces and is applied against the inner surface of the part. In this way, couple can be transmitted between the shaft and the part through the conical assembly which, to do this, uses the principle of conical force-fitting.
However, this solution has the drawback of only being applicable to massive parts such as solid shafts and large hubs.
Another known embodiment described by French patent FR-2405 386 consists in using a cone working on wedges applying to a tubular part while also being integral with another tubular part. This solution is suitable for transmitting relatively low torque between two parts, but certainly not for transmitting high torque, as required for the tail rotor of a helicopter.